The design of the premixed gaseous engine began with the injector. The successful operation of the premixed injector is the primary goal of this thesis. It is important the propellant gases are thoroughly mixed and then injected at high velocities into the combustion chamber. The first section is the manifold section used to insure the gases are thoroughly mixed where the gases are first introduced in an orthogonal fashion. The oxygen was be injected into the primary flowpath and the ethylene injected at four ports orthogonal to each other and the oxygen flow. The injection of the ethylene should help mix the individual flows as it is injected equally spaced around the core. The mixing will take place as the gases flow the length of ten diameters. The flow will travel four inches in a 0.4 inch diameter section. According to Kuo this will suffice for proper mixing. 3 A smaller port placed upstream is for a burst disk to relieve pressure in the manifold section in case of flashback. If combustion travels upstream beyond the injector into the mixing chamber the propellants could detonate. The burst disk diverts this pressure out of the engine. This part was machined from oxygen-free copper to dissipate heat in the case of flashback.
Figure 24. Pictures of the Injector Manifold to thoroughly mix reactants
The next injector section was the Diffuser Chamber which had a diverging cross section with an eight degree half angle to the injector face. The diverging section allows the flow velocity to decrease, decreasing the pressure prior to the injector. Both the Injector Manifold and Diffuser Chamber were manufactured of oxygen-free copper allowing the best conductivity possible in the event that flashback did occur. The inertia conductivity of copper would be able to absorb the heat of combustion until the test was aborted due to flashback. A small port is placed three quarter inches behind the cooled premixed injector face for a thermocouple. The thermocouple was used as a ‘Flashback Trigger’ which would abort the test if the thermocouple detects a flame. An Omega exposed junction K-type thermocouple was used since they are more sensitive to detect this flame. The abort condition was set 14 degrees above the ambient temperature of the cold gas flow and was our primary means to detect flashback. The Normally Closed
leads from the Omega Programmable Digital Thermocouple Meter (DP25-TC) are wired in series with the Test Cell Three ‘Emergency Stop Button.’
Figure 25. Pictures of the Diffuser Chamber with a 16 degree diverging section The premixed injector section is a water-cooled injector that is bolted on between the injector manifold and the combustion chamber of the engine. The injector consisted of seven injector tubes that inserted into two injector faces. These faces were sandwiched together with the tubes in between. The tubes have O-ring seals to ensure the water coolant does not come in contact with the propellant effluents. If this were to occur the water could possibly vaporize to steam and cause hardware damage. The sandwiched injector is place in the Injector housing where it is also sealed by O-ring seals. The seven injector holes are 1/32 inch in diameter and have the ability to be modified to a larger diameter to allow larger mass flows and higher chamber pressures if deemed appropriate. The injector tubes are long in relation to the orifice diameter so in the event of a
flashback the flame has a greater length to travel along the cooler wall. The Injector was also machined from oxygen free copper and the pieces are relatively thick for heat conduction. The water passages around the injector tubes were kept as a large of volume as possible to allow the water to effectively cool the injector tubes and faces. The water output will monitored to detect evidence of flashback as well. Combustion within the injector ports would be indicated by the increase in water temperature. The temperature datum was taken at the beginning of the run prior to ignition. The abort condition was set 8 degrees above the temperature datum. The downstream face will be in contact with the combustion chamber; therefore this is the least sensitive indication. The datum temperature will be the temperature water output prior to ignition. As standard procedure the water cooling will be turned on minutes prior to each shot to ensure the copper has reach thermal equilibrium.
Figure 26. Pictures of the Cooled Premixed Injector
The injector orifices are small diameter smooth-bored holes resulting low Reynolds numbers. This maintains a laminar flow inside the injector with Reynolds numbers less than 2000. The Reynolds is calculated using Equation 2 below.
3
Re
Diameter of tube (m)
Average velocity of fluid (m/s) Density of fluid (kg/m ) Viscosity of fluid (m s /kg) lU l U ρ µ ρ µ = = = = = i (2)
Another design parameter of injectors is the discharge coefficient. The discharge coefficient defines the injector’s ability to maintain the mass flow through the injector with a minimum of pressure drop across the injector face. High value of discharge coefficients (maximum value of 1.0) are smooth and have well-rounded entrances. These are the most common. The discharge coefficient of our injector is predicted as 0.82. This can be calculated using Equation 3 below.
2 2 3 2 Mass flow (kg/s) Discharge Coefficient Area of Orifice (m )
Pressure drop across orifice (N/m ) Density of propellant mass flow (kg/m )
d d p m C A m C A p ρ ρ = = = = = = ∆ ∆ (3)
The full assembly of the injector is shown below. The mixing section and the diverging section are rather large and bulky, but can likely be reduced to simple hardware once the premixed injector is better characterized. The three inch combustion chamber and nozzle with 0.3 inch diameter throat was bolted on following the injector. This combustion chamber and nozzle were used and applied as a constant throughout this research.
IV. MODELING
A. SIERRA ENGINEERING PREMIXED INJECTOR
The gaseous flow through Sierra Engineering’s premixed injector was modeled using Computational Fluid Dynamics (CFD) software designed by CFDRC and sold by the ESI Group. The geometry of the injector was modeled in three dimensions and meshed using CFD-GEOM where the file can be imported into the appropriate solver. The solver used was ESI Group’s CFD-ACE+ to compute the fluid flow and mixing through the mixing chamber. The modeling was completed to understand the gaseous flow through the swirling mixing chamber and to evaluate if the propellants were properly mixed prior to the bead field. This model was created using 500,000 cells.
Figure 28. Three dimensional CFD-GEOM model of Sierra Engineering’s Premixed Injector
The boundary condition models consisted of a symmetry boundary, wall boundaries, an inlet boundary and an outlet boundary. Once the models were complete they were exported to a *.DTF file that can be read by a solver program. The initial condition and constraints were then entered into the CFD-ACE+ solver and are listed in Appendix A.